Recently, in each of companies, universities and so on, the development of an electronic paper is actively promoted. Various applied methods of the electronic paper are proposed such as a digital book which is first on the list, a sub-display of a mobile terminal device, a display unit of an IC card. There is a display method using a liquid crystal composition in which a cholesteric phase is formed (cholesteric liquid crystal) as one of the display methods of the electronic paper. The cholesteric liquid crystal is also called a chiral nematic liquid crystal. This cholesteric liquid crystal is a liquid crystal in which molecules of the nematic liquid crystal form a spiral cholesteric phase by adding relatively large amounts (tens of %) of additive agent with chirality (chiral material) to the nematic liquid crystal. The cholesteric liquid crystal has the superior characteristics, such as a characteristic (memory characteristic) of holding a display semipermanently, a vivid color display characteristic, a high contrast characteristic, and a high resolution characteristic.
More specifically, the cholesteric liquid crystal includes bistability (memory characteristic), and enters into any one of a planar state, a focal conic state and a middle state where they are mixed, by the adjustment of an electric field intensity applied to the liquid crystal. Once the cholesteric liquid crystal becomes in the planar state or the focal conic state, the cholesteric liquid crystal is stable under no electric power afterward and maintains the state.
The planar state is acquired by making the electric field into zero rapidly after a given high voltage is impressed and a strong electric field is applied to the liquid crystal. On the contrary, the focal conic state is acquired by making the electric field into zero rapidly after a voltage lower than the above-mentioned given high voltage is impressed and the electric field is applied to the liquid crystal, for example. The middle state where the planar state and the focal conic state are mixed is acquired by making the electric field into zero rapidly after a voltage lower than the voltage for acquiring the focal conic state is impressed and the electric field is applied to the liquid crystal, for example.
When the cholesteric liquid crystal is in the planar state, a light of a wavelength corresponding to a spiral pitch of the liquid crystal molecules is reflected selectively. At this case, the center wavelength λ of the reflected light can be expressed by the product of an average refractive index n of the liquid crystal and a spiral pitch p (n×p). That is, the color display can be achieved by stacking two or more cholesteric liquid crystals, adjusting the value λ of each layer, and setting the reflected light of each layer to red, green and blue. The cholesteric liquid crystal that enables such a color display is disclosed by Japanese Laid-open Patent Publication No. 2002-116461, for example.
On the other hand, when only a single liquid crystal layer is used as the cholesteric liquid crystal, a percentage of the light that can be used for the display among incidence lights which enter into the liquid crystal layer is 50%. This is because the cholesteric liquid crystal has a characteristic to reflect either of a right-handed circularly polarized light component and a left-handed circularly polarized light component among the incidence lights. In order to use both of the right-handed circularly polarized light component and the left-handed circularly polarized light component of the light of the same wavelength for the display, the structure stacking a liquid crystal layer in which the spiral arrangement of the liquid crystal molecules is counterclockwise (hereinafter referred to as “L layer”) and a liquid crystal layer in which the spiral arrangement of the liquid crystal molecules is clockwise (hereinafter referred to as “R layer”) is disclosed by Japanese Laid-open Patent Publication No. 2003-29301.